Project managers with appropriate permission can't restore archived workspaces
Why can't project manager with all appropriate permissions including 'Can Administer Settings' permission not able to unarchive a workspace?
This is working as expected. To unarchive a workspace, a user must be an administrator with the 'Workspaces' and 'Can Administer Settings' permissions. The reason why users require the 'Workspaces' permission is because you're essentially creating a new workspace.